Recently, importance on development of next-generation clean energy sources has increased due to the depletion of fossil fuel reserves. Thereamong, a solar cell is a device that converts solar energy directly into electricity. Solar cells can serve as an energy source to solve energy problems in the future, since they do not emit pollutants, have semi-permanent lifespan and utilize unlimited energy from the sun.
Solar cells may be divided into a variety of kinds depending upon materials used in a light-absorption layer, and the most currently available solar cell is a silicon solar cell. However, since silicon prices have been rising due to shortage of high purity silicon, a thin film type solar cell is drawing attention. The thin film type solar cell is fabricated to a thin thickness and thus contributes to less material consumption and light weight, thereby providing a wide application range. Copper Indium Gallium Selenide (CIGS) having high optical-absorption coefficient has been in the spotlight as a material for such a thin film solar cell. With the use of CIGS in the manufacture of solar cells, higher conversion efficiency can be obtained.
On the other hand, there is rising interest in tandem-structure solar cells presented as a solution to further improve efficiency of CIGS solar cells. The tandem-structure solar cell refers to a solar cell that has a multi-layer structure in which two unitary CIGS solar cells are stacked. However, since the tandem-structure solar cell is fabricated by first forming a bottom cell and then forming an upper cell thereon, the previously formed bottom cell can be damaged in the formation of the upper cell, thereby making it difficult to obtain sufficient energy-conversion efficiency.
Korean Patent Publication No. 10-2009-0065894A discloses a method of fabricating a tandem-type CIGS solar cell in which a metal nanoparticle layer is interposed between unitary cells to decrease an energy barrier at an interface between cells. However, this technique does not solve the problem of the previously-formed bottom cell being damaged in the process of forming the upper cell.